Copper wires are of increasing importance in fine elements in which delays in circuits due to parasitic resistance and parasitic capacitance of wires are dominant since the copper wires can achieve a lower resistance, a lower capacitance, and higher reliability than those of aluminum wires. As a typical method of forming copper wires, damascene processes are widely accepted. Of the damascene processes, a dual damascene process is accepted in view of manufacturing cost. With the employment of this dual damascene process, it has been expected that a copper wire process would accomplish a lower cost than that of a conventional aluminum wire process.
However, since copper is prone to oxidization as compared with aluminum, it has been necessary to use silicon nitride (SiN) with a high relative dielectric constant of 8 as a cap film to prevent oxidization of the surface of the copper. As a result, a detriment of increasing parasitic capacitance in the overall wire system occurs. In addition, since the surface of the copper is chemically unstable although the copper has properties which raise expectation of a high electromigration resistance, an interface between the copper and the silicon nitride acts as a path on which the copper is preferentially diffused to cause a problem of failing to obtain the expected high electromigration resistance (reliability).